Traffic-signal actuators — some documentation

As described in another post on this blog, metal detectors embedded in the street to trigger traffic signals first appeared around 1970, when transistorized electronics made them practical. By 1986 they were in fairly widespread use, and the definitive report on them, the City of San Diego Traffic Signal Bicycle Detection Study, had been published.

I myself have published on this topic.

Two articles in the June, 1983 issue of Bicycling Magazine, one of which I wrote, describe a fatal crash in California: a bicyclist could not trigger a protected left-turn signal phase. He was struck and killed by a vehicle proceeding straight through in the opposite direction, concealed by another vehicle in the opposite-direction left-turn lane. The protected left-turn phase was supposed to be triggered by a metal detector buried under the surface of the street. The metal detector was not sensitive enough to detect a bicycle. This incident likely prompted the work which led to the San Diego report.

I also posted an article about traffic-signal actuation issues on my Web site in 1993, and have updated it several times since.

The measures described in the San Diego report are easy to implement: they require only a change in the pattern of wires installed in the street, and an adjustment of the sensitivity of the electronic controller. A painted marking to indicate the best place for a bicyclist to wait is desirable with some wiring patterns. No new equipment is necessary. Though there have been many other publications about traffic-signal actuation, adoption of the measure has been spotty.

Here is a selection of current reference materials available online as of November, 2018:

The 2012 edition of the AASHTO Guide for the Development of Bicycle Facilities, section 4.12.5 (page 4-47 ff.) explicitly indicates that

[a]ctuated traffic signals should detect bicycles; otherwise, a bicyclist may be unable to call a green signal and may be forced to break the law by violating a red signal. Various technologies are available for detecting bicycles, including inductive loops, microwave, video, magnetometers, and pushbuttons.

The AASHTO Guide deprecates the quadrapole loop in favor of the diagonal loop, which is sensitive across its entire width and so, does not require a pavement marking .

The Manual on Uniform Traffic Control Devices describes a sign and pavement marking to use in conjunction withe loop actuators.

Portland State University has published a detailed technical report which addresses issues of clearance times etc., Operational Guidance For Bicycle-Specific Traffic Signals in the United States.

Some documents from advocates and local governments, among many others:

• Bikewalk NC description of actuators including newer types (video detectors, etc.)
• City of Santa Cruz, California, description of measures for bicycle actuation
• Not just a problem for bicyclists: “What to do about traffic signals that ignore motorcycles”

Steven Goodridge, North Carolina bicycling advocate and engineer, earlier posted an article describing the technical functioning of actuator loops in terms understandable by a layperson.

In recent years, as described in some of these documents, video detection has become common, though it can fail in the presence of heavy rain, snow or falling leaves. At night, a bicyclist may need to aim a headlight directly at the camera. LIDAR (Light detection and ranging) sends out light pulses to measure the distance as well as the direction of objects, and overcomes the problems with video, but as of yet is expensive and has seen little use for traffic-signal actuation. (It is a keystone tehcnology for driversless vehicles.)

Video and LIDAR vehicle detectors  respond to bicycles which have non-metallic (usually carbon-fiber composite) rims and tires with non-metallic (usually Kevlar) bead wires. Loop detectors do not. This problem should be addressed in product design regulations — the province of the U. S. Consumer Product Safety Commission) by requiring a loop of wire in the rim, but the CPSC has not addressed it.